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He said the only prominent American who did not support the UK remaining in the 28-member bloc was Republican presidential hopeful Donald Trump.

Mr Clegg said during his time in government he was told "over and over again by US decision makers" that British influence in Brussels "strengthened the West" and the co-operation between the EU and Nato.

He highlighted recent interventions in support of Britain’s EU membership by Washington's ambassador in London Matthew Barzun, secretary of state John Kerry and former State Department spokesman James Rubin.

He added: “And, most importantly, President Obama has said that having the UK in the European Union gives the States 'much greater confidence about the strength of the transatlantic union'.

“It is a case I hope and expect he will make loudly and clearly when he visits London in the coming days.

"In fact, the only prominent American who has dissented from that view is Donald Trump.

Trump wants Britain out of the EU; President Obama wants us to stay in

Nick Clegg

"So, the choice for any British voter who cares about American opinion couldn't be clearer: Trump wants Britain out of the EU; President Obama wants us to stay in.

“It is one of the more remarkable twists in this ongoing saga that Boris Johnson, a contender for the future leadership of Britain's most Atlanticist political party, should side with Trump rather than Obama. I know who's side I - and I suspect millions of other Brits - would rather be on."

The former Deputy Prime Minister said: "Britain and the United States will always be cousins, but we may not always be partners. If we leave Europe behind, America may leave us behind too.

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Clegg said Donald Trump is the only prominent American who does not support the UK staying in the EU

"By walking away from the top table of European affairs, we cede our influence and our importance.

"That's why I believe that leaving the European Union will have an unravelling effect on all our most important relationships. With our neighbours in Europe. With our cousins in America. And even with our brothers and sisters within the United Kingdom if it pushes Scotland closer to independence.